Imaging devices, such as optical scanners, are well-known in the art and produce machine-readable image data signals that are representative of a scanned object, such as a photograph or a page of printed text. In a typical scanner application, the image data signals produced by an optical scanner may be used by a personal computer to reproduce an image of the scanned object on a suitable display device, such as a CRT or a printer.
A hand-held or portable optical scanner is an optical scanner which is designed to be moved by hand across the object or document being scanned. The hand-held scanner may be connected directly to a separate computer by a data cable. If so, the data signals produced by the hand-held scanner may be transferred to the separate computer "on the fly," i.e., as the image data are collected. Alternatively, the hand-scanner may include an on-board data storage system for storing the image data. The image data may then be downloaded to a separate computer after the scanning operation by any convenient means, such as via a cable or an optical infrared data link.
Hand-held or portable optical scanners are well-known in the art and various components thereof are disclosed in U.S. Pat. No. 5,552,597 of McConica for "Hand-Held Scanner having Adjustable Light Path", U.S. Pat. No. 5,586,212 of McConica, et al., for "Optical Wave Guide for Hand-Held Scanner," U.S. Pat. No. 5,381,020 of Kochis, et al., for "Hand-Held Optical Scanner with Onboard Battery Recharging Assembly," and U.S. Pat. No. 5,306,908 of McConica, et al., for "Manually Operated Hand-Held Optical Scanner with Tactile Speed Control Assembly," all of which are hereby incorporated by reference for all that they disclose.
A typical hand-held optical scanner may include an illumination system and an optical system to accomplish scanning of the object. The illumination system illuminates the object and the optical system collects light reflected by the illuminated object and focuses a small area of the illuminated object, usually referred to as a "scan region" or "scan line," onto the surface of a photosensitive detector positioned within the scanner. Image data representative of the entire object then may be obtained by sweeping the illuminated scan line across the entire object, usually by moving the hand-held scanner with respect to the object. By way of example, the illumination system may include a light source (e.g., a fluorescent or incandescent lamp or an array of light emitting diodes (LEDs)). The optical system may include a lens and/or mirror assembly to focus the image of the illuminated scan line onto the surface of the detector.
The photosensitive detector used to detect the image light focused thereon by the optical system may be a charge-coupled device (CCD), although other devices may be used. A typical CCD may comprise an array of individual cells or "pixels," each of which collects or builds-up an electrical charge in response to exposure to light. Since the quantity of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots on an image focused thereon.
Many hand-held scanner devices include a power drive system to assist the user in moving the hand-held scanner device over the object being scanned. One type of power drive system may even include a speed control system to ensure that the user moves the hand-held scanner over the object within an appropriate speed range to ensure good scanning performance. Still another type of power drive system may incorporate a battery recharging system to allow an on-board battery to be recharged by the drive system, such as, for example, if the user attempts to move the hand-held scanner device over the object at a speed faster than the maximum scanning speed for the particular application.
Regardless of the particular features of the power drive system, most power drive systems are not without their problems and disadvantages. For example, one type of power drive system utilizes a relatively low torque drive motor. While such a low torque drive motor has the advantage of being physically small and relatively inexpensive, it requires a gear box or transmission assembly having a high reduction ratio (typically involving several stages of spur gears) to provide the relatively high torque required by the drive roller. Unfortunately, high-reduction ratio transmission assemblies typically involve a high parts count, increased assembly time, and cost. In addition, such transmissions may have relatively low efficiencies. Such high reduction ratio transmissions may also include a significant amount of backlash which can complicate position tracking and speed regulation, if such systems are provided.
Partly in an effort to eliminate some of the problems associated with the foregoing power drive systems, other types of drive systems have been developed that utilize high torque motors. Such high torque motors are capable of providing the required torque to the drive wheels or rollers, but without the need for transmissions having large reduction ratios. Unfortunately, however, the high torque motors are usually physically larger, more expensive, and usually produce more noise than low torque motors. Another problem is that the motors draw more current, which means that the electronic control system which supplies power to the motor must have a higher current rating.
All in all then, both types of drive systems involve various trade-offs and neither type of power drive system has a clear advantage over the other. Consequently, a need remains for a power drive system for hand-held scanner devices that is relatively small and compact to allow the overall hand-held scanner device to be made as small as possible. Additional advantages could be realized if such a compact drive system included a minimum number of components, but without the need for relatively large, high-torque drive motors or for transmission systems involving multiple stages.